This article is about the flow of fluids in biological systems. For use of mass flow in physics see Fluid Dynamics.
For bulk flow in physics see Advection. For protein transport in cell biology see Bulk movement. As such, mass flow is a subject of study in both fluid dynamics and biology. Examples of mass flow include blood circulation and transport of water in vascular plant tissues. Mass flow is not to be confused with diffusion which depends on concentration gradients within a medium rather than pressure gradients of the medium itself.
In general, bulk flow in plant biology typically refers to the movement of water from the soil up through the plant to the leaf tissue, but can also be applied to the transport of larger solutes e. According to cohesion-tension theory , water transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via hydrogen bonding combined with the high water pressure of the plant's substrate and low pressure of the extreme tissues usually leaves.
Nobel et al. Trolinder et al. However, there have been no broad comparisons of fruit transpiration rates and vascular functioning.
Another possibility is that the xylem acts to buffer any imbalances between transpiration, growth and more constant inward flows of phloem water Choat et al. Continuously circulating or temporally oscillating flows have been measured for several species including kiwifruit, Actinidia chinensis Planch. Clearwater et al. In this species, models show that the amount of inward flow in the xylem decreases with decreasing assumed phloem sap concentration, but within the likely range of phloem concentrations, there is always a requirement for some inward xylem flow Fig.
However, it is possible that variation in the overall contribution of the phloem to fruit development is correlated with interspecific differences in phloem sap concentration. Another factor that contributes to changes in fruit hydration during development is phloem unloading between sieve elements and sink tissues.
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The transition to apoplastic unloading occurs primarily in sinks that accumulate high concentrations of osmotically active solutes such as ripening grapes and tomatoes Lalonde et al. However, more research is needed to understand whether the form of sugar unloaded in the fruit e. By combining our knowledge of flowers and fruit, a model emerges of highly regulated changes in phloem and xylem functioning, coordinated with each stage of reproductive development.
The phloem contribution to hydration is expected to increase with decreasing phloem sap concentration, organ transpiration and fresh weight growth, and with increasing carbon requirements for nectar or storage. However, more research is needed to determine whether the balance between the two vascular tissues differs between species, environments and functional groups and whether hydraulic isolation in reproductive organs is favoured in warmer and drier environments or in taxa that accumulate high levels of osmotically active solutes in their petals or ovaries. Just as changes in xylem functioning have had a major role in the evolution of terrestrial plants Boyce et al.
Models of carbon allocation in trees and carbon cycling in ecosystems require information about the rate and size of carbon fluxes that occur in plants. Although several ecosystem e. Bonan et al. A more mechanistic understanding of how canopy photosynthesis and transpiration fluxes are coupled to water and carbon use including plant and soil respiration and phloem transport and the factors that control changes in carbon allocation within plants would make it easier to model the rates of ecosystem carbon and water exchange and examine how sensitive surface fluxes will be to environmental forcing McDowell et al.
Although this type of analysis does not explicitly deal with the phloem, its estimates of carbon flux are based on assumptions about phloem transport. Here, we highlight two critical aspects of phloem transport that have important implications for whole plant carbon balances: the variety of carbon sources that exist in the plant and the possibility of flow in multiple directions not exclusively from the leaves to the roots.
Carbon supply via photosynthesis is sensitive to multiple factors such as light, temperature and tissue hydration, and hence, the availability of fresh assimilates for various plant processes is often interrupted. The role of NSC as a carbon source further increases towards spring, when new growth takes place before leaves are fully expanded and active.
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More than half of each of these pools was degraded and consumed during the transition from wet to dry season, indicating the important role of starch as a carbon source for plant activities outside leaves. These results reinforce that fact that carbon loading into the phloem can take place at multiple sites along the transport pathway, depending on the availability of mobile carbon in adjacent cells. Tree carbon balances also need to consider that the direction of phloem transport can change depending on source and sink activity.
These flows were relatively minor in size ca. The potential carbon relocation to the leaves in July correlated with a minimum in the foliage starch pool, while the stem starch pool was also close to its lowest level. This stem to foliage relocation suggests some low threshold level at which local starch hydrolysis stops and carbon must be imported into leaves, in agreement with Hoch Despite growing interest in understanding patterns of carbon allocation within plants, relatively few studies have attempted to make a direct connection between carbon allocation and phloem transport but see Nikinmaa et al.
Future experiments considering the dynamic nature of phloem transport will allow us to better understand to what extent relocation flows can change e. Many aspects of ecosystem function, plant growth and productivity are influenced by plant size including carbon storage, canopy transpiration and plant water usage e. Falster et al. As a plant grows, it can gain better access to water and light by increasing the footprint of its canopy and root system, but it also has to invest more resources in structural and supportive tissues.
At the same time, it experiences more negative water potentials, lower hydraulic conductivity and lower stomatal conductance in its canopy. Givnish ; Givnish et al. McCulloh et al.
Earlier modelling by Tyree et al. The main effect of increasing sieve tube conductivity in longer tubes i. They showed that under many conditions, water potential equilibrium between xylem and phloem was satisfied, and carbon transport could occur from sources to sinks in an osmotically regulated fashion see section on Carbon—Water Interactions. However, in this model, the behaviour of the sieve tube is strongly dependent on tube radius and length, such that in a very long tube tens of metres significant declines in phloem turgor are predicted to occur, potentially impairing carbohydrate transfer in tall trees Thompson However, similar to the situation for the xylem e.
While we still do not have a clear understanding of the relative contribution of all the possible components, one can speculate that many factors influence carbon transfer efficiency and reduce resistance along the transport pathway. Some of these factors include the progressive widening of sieve element diameters, the lengthening of sieve elements, which will increase the mean distance between sieve plates, the decrease in conduit frequency and density and the decrease in phloem area relative to xylem area towards the base of the plant e.
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The effects of gravity on axial phloem transport and on the scaling of phloem anatomy with plant size have generally not been considered carefully in tall plants despite the analytical solutions that exist for both xylem and phloem transport Jensen et al. Gravity can impact phloem transport in two very different ways, that is, 1 by affecting turgor at the top of tall plants and therefore the realized cell size as a consequence of the effects of turgor on cell growth and 2 as an additional force that affects the movement of a solution along a vertical axis.
Effect 1 impacts the scaling of both xylem and phloem conductance as a consequence of parallel changes in apical cell diameters. Effect 2 hinders the upward pull in the xylem and phloem but helps transport of carbon towards the roots. While much progress has been made in elucidating mechanisms of phloem loading and unloading, questions about the physical organization of this vertical superhighway, and the significance of plant height for phloem transport, have not been explored thoroughly.
We still lack a sound understanding of how environmental factors affect these anatomical and morphological traits, how this set of characters relates and trades off against other sets of plant traits and how these strategies combine and vary across evolutionary lineages to affect carbon transfer at the ecosystem scale. Plant carbon use has large scale implications for ecosystem function because of the role that plants serve in carbon sequestration Barford et al. Plants transfer carbon belowground by two pathways, that is, via the detritus formed by leaf and root litter and by mass transport in the phloem see next section on Biotic Interactions.
The detritus feeds heterotrophic organisms predominantly fungi and bacteria and is therefore primarily responsible for the dynamics of carbon stocks present in soils and litter layers e. Malhi et al.
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Typically, the timescale for the production of this detritus is in the order of years to decades. Continuous measurements of soil CO 2 efflux and vertical soil CO 2 profiles, coupled with measurements of canopy photosynthesis by eddy covariance, have allowed the exploration of the diel and seasonal dynamics of the links between carbohydrate production and consumption by autotrophic and heterotrophic respiration in the field reviewed by Vargas et al. For example, Vargas et al.
Another approach used to determine the significance of phloem transfer to carbon cycling has been to curtail this transfer directly, either by root trenching cf. Using the latter approach, Dannoura et al. This means that both the transport rate of the phloem and the architecture of trees could have important implications for carbon cycling. In general, the rate of carbohydrate flux must be equivalent to canopy photosynthetic flux, the net of the fluxes going into and coming out of intermediate storage pools and sinks present along the transport pathway see section on What Assumptions Are Made About Phloem Transport in Tree Carbon Budgets?
This inventory approach can also be extended to incorporate the carbon fluxes to the soil. Using this method over typical intervals of one to several years, one arrives at reasonable estimates of phloem fluxes belowground. For example, Giardina et al.
Accurate models of belowground carbon transfer rely on a clear mechanistic understanding of the extent that phloem transport dominates carbon flux into the soil and how coupled carbon transport is to changes in environmental conditions.
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